The present invention relates to an aqueous cleaning solution and a method for leaning aluminum-based metals. More specifically, the present invention relates to an aqueous cleaning solution and a method for cleaning aluminum-based metals characterized by excellent cleaning action in the removal of lubricating oil, aluminum powder and the like adherent to the surface of aluminum. Further, the present invention relates to an aqueous cleaning solution lessened influences on living things and environments, and a method for cleaning aluminum-based metals thereby.
Manufactured articles having aluminum surface, for example, beverage containers made from an aluminum-based metal (i.e., aluminum or an aluminum alloy), are fabricated by a forming operation which is usually called drawing and ironing (hereinafter referred to as DI processing). Lubricating oil is applied to metal surface in this forming operation. Further, aluminum powder or a reaction product (i.e., smut) between the aluminum powder and the lubricating agent adheres to the containers obtained and to inner walls thereof in particular. Later, usually these containers are subjected to a chemical conversion treatment or coated with a paint for surface protection. In order to conduct these treatments uniformly and perfectly, it is necessary to cleanse the surface by removing the lubricating oil and the smut from metal surface prior to the chemical treatment.
In the surface cleansing process, an acid cleaning agent, which cleans by appropriately etching metal surface, is generally used in order to remove an oxide film and the like formed on the surface of aluminum. Heretofore, a chromic- or hydrofluoric acid-based cleaning agent has been often used as the acid cleaning agent. These cleaning agents contain harmful substances, however. Therefore, in recent years, it has been desired the establishment of a chromium-free and fluorine-free acid cleaning technique which is performed at a lower temperature.
Japanese Patent Application Publication (JP-B) Nos. 3-50,838 and 3-65,436 propose a chromium-free and fluorine-free acid cleaning technique.
Disclosed in these publications are an acid cleaning agent which contains a little or no fluorine ion and has a pH value adjusted to 2 or less by sulfuric acid and/or nitric acid and which further contains a ferric ion in place of the fluorine ion for the acceleration of etching, and a controlling method in which the concentration of the ferric ion in the cleaning bath is controlled by controlling the oxidation-reduction potential of the bath.
Normally, the aluminum etching reaction in an acid cleaning agent consists of an anodic reaction in which aluminum becomes aluminum ions (Al.sup.3+) and an cathodic reaction in which H.sup.+ in the cleaning solution is reduced to become 1/2 H.sub.2. Therefore, if ferric ions (Fe.sup.3+) are added to the acid cleaning solution, an anodic reaction, in which (Fe.sup.3+) is reduced to Fe.sup.2+, takes place concurrently with the reduction of H.sup.+. As a result, the etching reaction of aluminum is accelerated.
Further, control of the oxidation-reduction potential of the cleaning bath by the oxidizing agent makes it possible to suppress the concentration of Fe.sup.2+, which increases as the etching reaction of the aluminum proceeds, and to oxidize Fe.sup.2+ to Fe.sup.3+.
Besides, Japanese Patent Application Laid-Open (JP-A) No. 7-173,655 discloses an aqueous acid cleaning solution comprising a chelating dispersant and a surfactant in addition to the inorganic acid and oxidized form metal ion, as an attempt to prevent the formation of precipitate derived from iron ions and to further improve the leaning properties.
However, in the cleaning methods using any of the above-described cleaning agents, the cleaning properties are degraded due to the accumulation of lubricating agents if cleaning operation is continued for a long period of time. Further, measures, such as addition of a large amount of surfactants or increase of the amount of auto-drain for the reduction of the accumulation of the lubricating agent, lead to the increase in running costs and increase of the load of waste water. Furthermore, an alkylphenol-based surfactant, which has been often used, presents environmental problems because it is suspected of being an environmental hormone and because its biodegradability is insufficient.
Still further, the addition of the oxidizing agent causes oxidative decomposition of the surfactant and the decomposed products accumulate in the acid cleaning solution, thus degrading the cleaning properties.